1. Field of the Invention
The present invention relates to an improvement in a thermal image recording system for reproducing a two-dimensional image in which one line of picture elements is printed on a thermal record medium by selectively energizing heating dots of a thermal head, the record medium is then shifted by one line and then the next line of picture elements is printed and so on.
2. Description of the Prior Art
A method for reproducing an image in a form of high quality hard copy comparable to a photograph (called a pictorial hard copy) from an electrical image signal supplied by television, video camera or electronic still camera has been extensively studied. One of the most effect methods is a thermal image recording method in which a thermal record medium (which includes not only a thermal coloring type medium but also a medium comprising an image sheet and a thermal transfer ink sheet) is moved on a linear thermal head having a line of heating dots and the heating dots are selectively energized in accordance with an input image signal so that one line of picture elements is thermally printed on the record medium, and the above operation is repeated for each line to sequentially print the lines of picture elements. FIG. 1 shows such a recorder. I denotes a thermally transferable ink sheet. A mixture of sublimative dye and binder is applied to an upper surface of the sheet. An image sheet R has an image layer, which accepts the sublimative dye and colors it, applied on a lower surface thereof. The thermal transfer sheet I and the image sheet R are contacted to each other and transported to the right as a platen P rotates. A thermal head TH selectively heats the sheet in accordance with an input image signal applied to a terminal E. At the heated areas, the sublimative dye on the thermal transfer ink sheet I is transferred to the image sheet R so that the image is printed thereon. The thermal head TH has fine dot heating elements HE arranged in a line normal to the plane of the drawing.
The number of heating dots corresponding to one line of picture elements may amount to 1280 depending on the size of the image. If the number of dots is so large and all elements in one line are to be printed in the same color, a maximum current which flows in one-line printing is very large and a high capacity power supply is required therefore.
As shown in FIG. 2A, it has been proposed to divide the heating dots into two blocks B.sub.a and B.sub.b and drive those blocks at sequential timing. In this method, the time required to print one line of picture elements is doubled but the maximum current required is reduced to one half.
When one heating dot is energized, it exhibits a temperature distribution as shown in FIG. 3. If all heating dots in one block are energized, a temperature at the boundary of the energized block B.sub.a (hatched dots in FIG. 4A) and the non-energized block B.sub.b (non-hatched dots) is low because of the presence of the low temperature non-energized block B.sub.b beyond the boundary and the heat dissipation thereto. This phenomenon is unavoidable. The same phenomenon occurs when the block B.sub.b is next energized, as shown in FIG. 4B. Accordingly, at the boundary of the blocks, the temperature is not sufficiently high in the first energization and the second energization (see FIG. 4C). As a result, the printed picture elements are discontinuous at the boundary and white dots appear. The white dots appear in the printed image as a white stripe (called a white line). Such a white line is a fatal defect when a high quality image comparable to a photograph is required.
An alternate block print system has been proposed, in which the dots are not divided into the blocks at the center of the line of dots but alternate dots are arranged in the respective blocks as shown in FIG. 2B. Namely, the odd-numbered dots belong to the block B.sub.1 and the even-numbered dots belong to the block B.sub.2, and the blocks B.sub.1 and B.sub.2 are alternately energized.
In this alternate system as well as other systems, in order to obtain a high quality print image comparable to a photograph, it is necessary to reproduce gray levels by controlling print densities of the picture elements, that is, amounts of coloring materials such as dyes to be transferred.
The number of picture element densities is an important factor. It is usually 16, 32, 64, 128 or 256. A most effective method to form the picture elements having such gray levels by a thermal head is to change a print (energization) time period. The print time and the density generally have a relationship as shown in FIG. 5.
The following method is usually used to control the print time. The time required to attain the highest density is divided by the number of gray levels to be reproduced or a higher number, with the sum of the pulse widths of the pulses being equal to the time required to attain the highest density. (The widths of the respective pulses are not necessarily equal.) A selected number of those pulses are applied to the dots of the thermal head to attain the desired gray levels.
In the proposed alternate system, the pulses are controlled up to the maximum gray level n (for example, n=64) in the first block B.sub.1, and then the pulses are controlled up to the maximum gray level n in the block B.sub.2, and these operations are alternately repeated. Accordingly, the block printing requires a print time of 2.times.T milliseconds, where T is the time to complete printing of one line of picture elements without block printing.
According to an experiment done by the inventors of the present invention for the proposed alternate block printing system, it has been found that the picture elements formed by the dots of the later energized block are darker than those of the earlier energized block because of a heat storage effect of the earlier energized block and a proximity effect. As a result, dark stripes appear in the final two-dimensional image on every other picture element line along the direction of movement of the record sheet.